In modern computer control systems, it is frequently necessary to convert a digital signal (which is used internally in the computer) to a variety of analog signals which are used to directly control or measure the environment. Two conversion devices which are often used in manufacturing systems are digital-to-analog converters (DACs) and analog-to-digital converters (ADCs). These units convert between analog signals generated by the environment and the digital signals used by the computer.
Another, perhaps less widely used, conversion device is a digital-to-time converter. This unit accepts a digital signal and produces a proportional time delay. The delay usually appears as a time difference between two pulses appearing at the output of the device or between a trigger pulse and a pulse appearing at the output of the device. Such programmable time delay circuits are often used in automated test equipment and are used to delay digital signals.
Digital-to-time converters have conventionally been fabricated from discrete semiconductor devices. In such devices, the conversion is often performed by comparing a linearly-increasing voltage or current ramp signal to a threshold voltage or current. In one conventional form of a digital-to-time converter, a fixed threshold voltage is set by a precision voltage reference source and the time delay is generated by comparing the threshold voltage to a ramp with a variable slope. The slope of the ramp is set by the value of the digital word to program the device. In another conventional form of the converter, a ramp with a fixed slope is generated and the time delay is obtained by comparing the ramp voltage to a variable threshold whose level is set in accordance with input digital word.
In either of the above variations, when the value of the ramp voltage equals the value of the threshold voltage a pulse signal is generated. If a pulse signal is generated at the start the ramp signal, the time elapsing between the two pulse signals represents a delay which depends on the value of the digital input word. The starting pulse may also be the trigger pulse which is used to start the ramp signal generation.
It would be convenient to fabricate a digital-to-time converter circuit as a monolithic integrated circuit. Such a device would have many obvious advantages over a discrete-device implementation of the same circuit. For example, the integrated circuit would be smaller, have higher reliability, better performance, a lower power consumption and a lower cost. However, practical problems are associated with the implementation of a digital-to-time device as a monolithic integrated circuit. One of these problems arises from the need to produce a device that is stable with variations in temperature and power supply voltages--a problem common with integrated circuits. The solution to temperature and power supply variation compensation problems generally involves the use of precision reference sources.
The first problem is to obtain a predictable ramp signal. In a digital-to-time converter designed with discrete devices, the internal ramp signal is conventionally generated by charging a capacitor with a stable current generated by placing a precision voltage reference source across a precision resistor. Such a precision voltage source is generally comprised of a voltage reference source, a resistor, and a control amplifier connected in a standard feedback configuration. Once a stable charging current has been established, the voltage across the capacitor provides a stable ramp output.
The second problem is to obtain a stable threshold value. In many prior art circuits, the threshold voltage is generated by a digital-to-analog converter (DAC). In order to assure predictable operation, the DAC voltage must also be referenced and controlled so that variations in the voltage caused by temperature and power supply changes track the temperature and supply-induced changes in the ramp voltage. In a typical prior-art design, the same voltage reference source used to generate the ramp signal is used to drive an additional control amplifier or a current mirror circuit to measure and reflect the ramp current into the DAC so that variations in the threshold voltage track variations in the ramp voltage.
This conventional approach requires the fabrication on the integrated circuit of a voltage reference source and control amplifier or a current mirror (which requires two different bipolar transistor types). In either case, the circuit becomes expensive and more difficult to manufacture.
The problem is additionally complicated because typically the resistor and capacitor used to generate the ramp voltage are external to the integrated circuit so that the user can easily change the ramp slope and, thus, the time constants involved in the circuit. However, the threshold voltage is generally determined by internal integrated circuit component values which may not track the temperature and supply changes in the external ramp components.
Accordingly, it is an object of the present invention to provide a digital-to-time converter which can be easily fabricated as a monolithic integrated circuit.
It is another object of the present invention to provide a digital-to-time converter which does not require the use of an internal voltage reference source and control amplifiers.
It is still another object of the present invention to provide a digital-to-time converter which can be manufactured entirely with transistors of one bipolar type.
It is yet another object of the present invention to provide a digital-to-time converter which is temperature and supply variation compensated to produce a stable output in spite of temperature and power supply variations.
It is still another object of the present invention to provide a digital-to-time converter which can be inexpensively manufactured.